1. Field of the Invention
The present invention relates to a recorded information reproducing apparatus and, more particularly, to a recorded information reproducing apparatus that reads out recorded information from a recording medium and reproduces the information.
2. Description of the Related Art
High-density information recording on a recording medium such as an optical disc can be realized by shortening a pit length and/or narrowing a track pitch. However, narrowing the track pitch of the optical disc causes crosstalk between signals on adjacent tracks when information is read out. Such a crosstalk prevents a read signal from having a desired waveform, so that a highly reliable reproduction signal cannot be obtained from the read signal.
In this respect, a recorded information reproducing apparatus may perform an adaptive signal processing for a cancellation of the crosstalk.
For example, a recorded information reproducing apparatus disclosed in Japanese Unexamined Patent Publication (Kokai) No. H9-320200 is designed to cancel a crosstalk by using signals read from three adjoining tracks with a 3-beam optical pick-up.
More specifically, a crosstalk canceler employs adaptive signal processing, which is performed by an adaptive digital filter based on, for example, an LMS (Least Mean Square) adaptive algorithm. First, intersymbol interference is removed from a read signal that has been read from a track T with the center beam spot to obtain a series of read sample values R. Then, two series of crosstalk sample values CR1 and CR2 corresponding to crosstalk components of the tracks adjacent to the track T are acquired through the adaptive signal processing on the basis of the read signals. A series of read sample values P free from crosstalk are obtained by subtracting the series of crosstalk sample values CR1 and CR2 from the series of read sample values R.
In this case, when three consecutive values in the series of the sample values P changes from positive to negative or from negative to positive, the middle of the three sample values, i.e., a zero-cross sample value, is extracted as an error signal E. The filter coefficient of the digital filter, in turn, is updated in accordance with the intensity of the error signal E.
In other words, when a read signal is waveform-equalized so as to meet the Nyquist""s second bandwidth requirement and is free of a crosstalk, the read signal crosses zero at the time it is sampled. When a crosstalk occurs, however, the waveform of the read signal is changed so that the read signal will not cross zero at the sampling time.
When a sample value at the zero-crossing time (i.e., zero-crossing sample value) is not zero, it is determined that a crosstalk corresponding to the error occurs. The conventional recorded information reproducing apparatus is constructed to update the filter coefficient of a variable coefficient filter so as to make the zero-crossing sample value corresponding to the error converges to zero.
Under certain circumstances for a crosstalk intensity, however, it may occur that the filter coefficients, which should be positive values in principle, are obtained as negative values, or that a calculation of the coefficients take a considerable time to converge. In the worst case, overflow in the crosstalk canceler may occur, thereby causing the canceler to operate unstable.
Accordingly, it is an object of the present invention to provide a recorded information reproducing apparatus having a crosstalk canceler capable of ensuring fast convergence of the filter coefficients of an adaptive digital filter and thereby achieving stable operation.
A recorded information reproducing apparatus according to the present invention comprises reading means for reading recorded information from one track on a recording medium to acquire a first read signal, and for reading recorded information from at least one track adjacent to the one track to acquire a second read signal; operating means for operating at least one coefficient on the second read signal; subtracting means for subtracting an output of the operating means from the first read signal; and coefficient setting means for setting at least one coefficient and for restricting at least one coefficient in such a way that the at least one coefficient becomes greater than a first predetermined value or smaller than a second predetermined value.